The present invention relates to a position detector for use with a mask and a wafer of an X-ray exposure apparatus or the like which employs a double-focus optical system utilizing chromatic aberration.
I have proposed an alignment position detector having a double-focus system for simultaneously detecting mark patterns on a mask and a wafer, for example, which are disposed at a minute interval, by employing a double-focus optical system utilizing chromatic aberration in Japanese Patent Application Sho Nos. 62-196174 and Sho 63-30600.
The principle of the double-focus optical system utilizing chromatic aberration will be described hereinafter with reference to FIG. 13.
In general, an objective has chromatic aberration to light rays of different wavelengths. For example, it has two different focal lengths to two kinds of g ray (wavelength .lambda.=435 nm) and e ray (.lambda.=546 nm). Accordingly, two images of a mark on the same point formed by g and e rays are different in position.
By way of example, an objective of a numerical aperture N.A.=0.4 and a magnification n=10 has focal lengths Fe=12.5 mm and Fg=12.74 mm and image point distances S'e=137.615 mm and S'g=137.5 m, to e and g ray, respectively.
When objects such as a mask and a wafer are disposed at points M and M' which are at a minute interval .delta. in a direction of the optical axis of the objective, as shown in FIG. 13, a mark on the mask point M forms images at points C and D which are at distance S'.sub.2 and S'.sub.3 from the objective on the optical axis and a mark on the wafer point M' forms images at points B and C which are at distance S'.sub.1 and S'.sub.2 from the objective on the optical axis to g and e ray, respectively, since g (solid line) and e (broken line) ray have different respective focal lengths Fg and Fe. Thus, images of marks at the mask point M and at the wafer point M' are formed at the point C, because of chromatic aberration, while the image of a mark on the mask at the point C is formed by the g ray and the image of a mark on the wafer at the point C is formed by the e ray.
Making observation now by disposing a detector such as a television camera at the point C, both marks at the mask point M and the wafer point M' can be seen at the same time but they are attended with blurs because of chromatic aberration. These blurs caused by chromatic aberration can be removed by employing a pattern barrier filter, so called, which is formed by combining filters transmitting g ray and cutting e ray and vice versa. It makes possible to observe images of alignment marks on the mask and the wafer which marks are at different positions, at the same point C at the same time.
The pattern barrier filter will be described briefly with reference to FIGS. 14 to 16.
The pattern barrier filter A has two wavelength zones, as shown in FIG. 16, for example: the central zone of a filter I which transmits e ray and cuts the rest wavelength-zone rays and both side zones of filters II which transmit g ray and cut the rest wavelength-zone rays.
When images are formed with a chromatic aberration objective mentioned above by superposing a square mask mark a on a rectangular wafer mark b whose width is smaller than that of the mask mark a, for example, as shown in FIG. 14, a sharp image a' of the mask mark a which is in focus to g ray and a blurred image a" of the mask mark a which is out of focus to e ray are formed on an image forming surface right and left, as shown in FIG. 15. In addition, a sharp image b' of the wafer mark b to e ray and a blurred image b" of the wafer mark b to g ray are formed in the middle. Making observation by superposing a pattern barrier filter A on the image forming point, the blurred image a" of the mask mark and the blurred image b" of the wafer mark are cut respectively by the filters II and I. As a result, only the sharp mask mark a' and the sharp wafer mark b' are simultaneously seen on the whole and it makes possible to precisely align the mask and wafer which are different in position.
An objective for use in the present invention is a chromatic aberration objective which positively causes chromatic aberration and properly compensates various aberrations.
Based on the principle described above, positional detection of high resolution is made possible by assigning two foci to surfaces of a mask and wafer which are disposed at a minute interval by utilizing chromatic aberration. This method, however, necessitates a pattern barrier filter, so that the scope of image formation may be limited and thus the visual field is reduced. In addition, speckles due to interference occurring within the pattern barrier filter or between the pattern barrier filter and the image pickup surface of a detector have a bad influence.
To eliminate the disadvantages, I have proposed an apparatus of removing speckles caused within a pattern barrier filter or between a pattern barrier filter and the image pickup surface of a detector by utilizing a wavelength-band ray in Japanese Patent Application Sho No. 63-30600. However, an essential object employing a wavelength-band ray is to prevent a multiple interference. It is also possible to eliminate speckles by coating a reflection preventing film on a pattern barrier film.
The principle of this proposal will be briefly described hereinafter. To make connection with the illustration in FIG. 13, let light rays in use be three of g ray (436 nm), e ray (546 nm) and d ray (587 nm). In the case that an image of an alignment mark on a wafer is formed by applying d and e ray to the wafer surface, the characteristics P of chromatic aberration as shown in FIG. 17 will be obtained by using a chromatic aberration objective designed and manufactured based on the principle described hereinabove. Assuming that an axial chromatic aberration between g and e ray (focus shift) on the mask surface is 40 .mu.m, this value corresponds to a gap .delta. between the mask and the wafer. It can be assumed that images formed by applying g ray to an alignment mark on the mask and e ray to an alignment mark on the wafer are at the same position.
Next, applying chromatic aberration compensation to d ray with an ordinary achromat to the chromatic aberration objective, so as to have an axial chromatic aberration Q shown in FIG. 18, the compensation of chromatic aberration to a wavelength-band ray in the neighborhood from e ray to d ray is made, so that an alignment mark on the wafer can be also detected at the same time with a required resolution. The condition .phi. for achromatization to e and d ray will be obtained as follows.
Let it be that the dispersion .nu..sub.1 of a concave lens constituting an achromat is: EQU .nu..sub.1 =(n.sub.1e -n.sub.1d)/(n.sub.1g -1),
the focal length of the concave lens to g ray is f.sub.1g, PA1 the dispersion .nu..sub.2 of a convex lens constituting an achromat lens is: EQU .nu..sub.2 =(n.sub.2d -n.sub.2d)/(n.sub.2g -1), PA1 and the focal length of the convex lens to g ray is f.sub.2g.
Then ##EQU1##
In addition, making consideration in the paraxial zone in a manner similar to FIG. 13, images formed by a wavelength-band ray including e and d ray are at the points C and D on the optical axis, as shown in FIG. 3.
In an apparatus employing the pattern barrier filter set forth hereinabove, when a light ray from a superhigh pressure mercury-arc lamp of comparatively high coherency (half width: 5 nm) is applied to a pattern barrier filter of a random surface configuration, the light ray is scattered and the phase of the wave front of the light ray is mondulated at random, resulting in the formation of a random pattern having bright and dark areas in light intensity, that is, speckles. Since the speckle pattern is found everywhere in the structure of optical systems such as a diffraction surface and an image forming surface of rays, even in the double-focus apparatus utilizing a chromatic aberration objective set forth hereinabove, images of reduced resolution may be detected in a superposed manner on a television image receiveing surface. In other words, when light rays of comparatively increased coherency, for example, rays from superhigh pressure mercury-arc lamp or laser rays, are applied to a pattern barrier filter, the phase of the wave front of the rays is modulated at random by refraction, reflection and transmission of the optical system and the rays reach a television image receiving surface which is an observation surface. In the observation surface, speckles are caused.
The most important point in the above description is that speckles are caused due to light interference. This means that without light interference, no speckle occurs. Interferences which will be caused in a pattern barrier filter can be eliminated basically by coating a reflection preventing film.
Consequently, when an illumination ray of wavelength-band is employed, which is the one selected out of rays as from a halogen light source and a xenon light source by a band-pass filter and which is an aggregation of incoherent rays whose wave fronts are irregular in phase, frequency and amplitude and thus have extremely reduced interference, the light interference phenomenon set forth above does not occur. If any, a change in light intensity is too slight to catch it with an eye and a camera. Thus, speckles adversely affecting the resolution do not occur.
This is the principle of the proposal in Japanese Patent Application Sho No. 63-30600 which improves the resolution by eliminating speckles occurring in the pattern barrier filter or between the pattern barrier filter and the image pickup surface of the detecter with an illumination wavelength-band ray. However, an essential object employing a wavelength-band ray is to prevent multiple interference in a resist film on a wafer.
Even with the improved double-focus apparatus utilizing chromatic aberration, however, the problems of reduction in the region of image formation and the field of observation are still left.
To solve the problems, it is an object of the present invention to provide a double-focus apparatus utilizing chromatic aberration which is capable of detecting a relative position between a wafer and a mask which are disposed at a minute interval with a high accuracy without the use of a pattern barrier filter.
Furthermore, other objects of the present invention are specifically given as follows.
.circle.1 A position detector is provided which does not use a pattern barrier filter which is comparatively difficult to manufacture and expensive.
.circle.2 Circumferential parts of image formation can be simplified and miniaturized to reduce a cost since no pattern barrier filter is employed to make installation and adjustment unnecessary.
.circle.3 No restrictive condition that the area of image formation is limited by configuration of a pattern barrier filter which is defined by the configuration of a mark is provided to obtain a positional detector of an increased observation field. Namely, the condition to the area of image formation based on a pattern barrier filter is eliminated, resulting in that two areas of image formation on focal planes make the entire area of image formation, which is an observation field.
.circle.4 Since no pattern barrier filter is employed, no limitation in configuration of a detection pattern barrier filter is placed to make detection with any detection pattern possible.
.circle.5 There is no speckle due to interference caused in a pattern barrier filter or between a pattern barrier filter and an image pickup surface which receives an images of a detection camera, so that resolution of a detection system is improved.
.circle.6 Vignetting areas are inevitably caused at both ends of the central portion of a pattern barrier filter in manufacturing, so that resolution is liable to be reduced. In the present invention, however, since no pattern barrier filter is employed, there is no reduction of resolution due to vignetting, so that an optical resolution power is improved.
In the case that a double-focus apparatus utilizing chromatic aberration of the present invention is applied to an alignment apparatus of X-ray aligner for use in the field of a semiconductor lighography, the present invention has the following specific objects.
.circle.a No pattern barrier filter is mounted around a taking picture lens (relay lens) and a position detector is substantially composed of an objective, a lens barrel and a television camera, so that the detector can be miniaturized, simplified in structure and reduced in weight as a whole.
.circle.b No pattern barrier filter is employed and, as a result, speckles and vignetting will be removed, so that resolution is optically improved and an accuracy of alignment is greatly improved.
.circle.c Since no pattern barrier filter is employed, limitations regarding configuration of an alignment mark are completely eliminated, so that an alignment mark of any configuration can be set. In the past, however, because of difficulty of manufacturing a pattern barrier filter, a conventional alignment mark can be detected only with a mark for the detection of one axis. With the present invention, it is possible to detect a mark of two axes, for example, a crossed mark.
.circle.d The number of alignment apparatus can be reduced. By way of example, while the detection of three axes, X, Y and .theta., has required to employ three detectors in the past, with the present invention it is possible to reduce detectors to two. Accordingly, in addition to the reduction of the number of alignment stages to be mounted on an alignment apparatus, it is possible to reduce a cost of the detector.
.circle.e The reduction of the number of alignment apparatus and alignment stages facilitates the dimensional design of the entire structure of the detector. In addition, the design condition of a mask stage is greatly lightened.
.circle.f No pattern barrier filter is employed and therefore there is no restriction of a catchup range (a visual field area of capable of detecting an alignment mark and making a positional measurement) which is given by a configuration of a pattern barrier filter. As a result, the scopes of fine alignment which is a fine position measurement and coarse alignment which is a coarse position measurement are greatly increased, so that misalignment (falling into a state of being unable to make alignment) (that is: an alignment mark gets out of place from a catchup range) is greatly reduced and throughput (the number of dealing with wafers per hour) is greatly improved.
.circle.g With a pattern barrier filter, a catchup range is largely reduced due to a configuration of the filter. (It depends upon magnification and, with my trial apparatus, is 15 .mu.m on the surface of an object with a magnification of 150 with respect to alignment of an X ray mask.) (Even with reduction in magnification for coarse alignment to 30, the range is 75 .mu.m, which is narrow.) As a result, it is difficult to share a detector in both alignments during a coarse alignment before a fine alignment, so that it is practically required for coarse alinment to further provide an optical system of a low magnification (5 to 30) and a low numerical aperture (N.A.=0.1). In contrast, since the present invention is not provided with a pattern barrier filter, the entire observation field becomes basically a catchup range. This fact is very convenient in coarse alignment of mask and wafer.
By way of example, when an apparatus for fine alignment is jointly used for coarse alignment, a catchup range is 15 .mu.m.times.(150/30)=75 .mu.m at the center of a pattern barrier filter with a magnification of 30 and this is insufficient for a catchup range in coarse alignment.
This means that a position of a wafer is shifted in the extent of 75 .mu.m before coarse alignment with a mechanical method such as a positioning pin and it is difficult to obtain a precise positioning. According to a calculation in this example, since no pattern barrier filter is employed, a catchup range makes the whole area of observation and is magnified from 15 .mu.m to 80 .mu.m which is a length of a short side of 1" image pickup tube. Accordingly, an observation field at this case is 80 .mu.m.times.(150/30)=400 .mu.m, which is about five times as large as that when a pattern barrier filter is employed and in area 5.sup.2 =25 times. This value may be sufficient for a range of the apparatus. Thus, the apparatus with no pattern barrier filter provided can be also shared in coarse alignment.
This makes it possible to conduct coarse alignment without the use of a detection apparatus which combines a microscope of a low numerical aperture for coarse alignment and an image receiving apparatus which are conventionally required. Thus, a double-focus apparatus utilizing chromatic aberration for fine alignment can be jointly used for coarse alinmnet. At this time, a required operation is only interchange of a taking picture lens. For example, a lens is interchanged by a revolver system to easily change a magnification of observation.
.circle.h This fact dispenses with the provision of a coarse alignment apparatus and an alignment mark is employed which makes a detection of two directions (X/Y) in the above .circle.c possible so that the number of required alignment apparatus can be reduced. By way of example, when three axes of X, Y and .theta. are detected by employing a conventional pattern barrier filter, it is required to provide total five units: three fine alignment apparatus of double-focus apparatus utilizing chromatic aberration and two coarse alignment apparatus which employ an objective of a low numerical aperture. In addition, it requires five alignment stages accordingly.
A double-focus apparatus utilizing chromatic aberration without a pattern barrier filter according to the present invention dispenses with a coarse alignment apparatus and can detect two axes of X-Y with a single alignment apparatus according to the above .circle.c . Thus, it is sufficient to provide only two alignment apparatus in total and this is a large improvement in operation. In addition, a dimensional design and the freedom of a mask stage which are severe in design are relieved. As a result, it is possible to make a design without any technical difficulties, resulting in reduction of a cost.
In addition, the rigidity of a mask stage including an alignment apparatus is increased to greatly affect the improvement of a final and overall accuracy in combination with an X-Y stage.
According to the present invention, a double-focus apparatus utilizing chromatic aberration for use in a positional detection apparatus for detecting a relative position between first and second object which are disposed at a minute interval, in a direction perpendicular to the interval direction, is characterized in that a vertical illuminator for simultaneously illuminating the first and second object in the direction of the interval by first single wavelength ray and second wavelength-band ray, including a band-pass filter or filters for transmitting said first and second ray, both or respectively and an objective for producing axial chromatic aberration corresponding to the minute interval with respect to the first single wavelength ray and the second wavelength-band ray, thereby simultaneously detecting a relative position between the first and second object in a direction perpendicular to the interval with rays reflected by the first and second object.
Consequently, it is possible to simultaneously detect a relative position between first and second objects which are disposed at a minute interval, in a direction perpendicular to the optical axis, without the use of a pattern barrier filter, resulting in that an alignment accuracy is improved because of the improvement in an optical resolution, a catchup range is increased and a positional detector apparatus can be simplified and miniaturized. In addition, the positional detector apparatus can detect a position without reducing resolution and a cost can be greatly reduced.